The present invention relates generally to the field of telecommunications. More particularly, the present invention relates, in one aspect, to access to communications system resources. Still more particularly, aspects of the present invention relate to control of access to mobile communications facilities.
Many communications systems, and systems employing communications as an integral part of their infrastructure, rely on access to available links and other facilities having at least some minimum available communications capacity. As a most common example, reliance is routinely placed on the availability of dial tone for placing traditional plain-old-telephone-service (POTS) calls. In the context of modem cellular and personal communications systems, the availability of an access channel plays an analogous role. In yet other circumstances, ongoing processes at a distributed system node rely on responses from other processor-based systems, such as database systems or special-purpose processor nodes.
When expectations for capacity or availability are not immediately fulfilled, e.g., because of link or system capacity limitations, service seekers frequently repeat requests for service. Such repeat requests add to the demands placed on the capacity-limited link or element, and service demands increase still farther, potentially to the point that little, if any, service capacity is available for any requester.
Many congestion control techniques have been employed in the public switched telephone network for dealing with an overload of POTS calls originating at or near (or directed to) the location of public emergencies, natural disasters and the like. Many of these techniques seek to limit or throttle originations (or terminations) of calls from (or to) the affected area. For example, it has long been a practice in administering telecommunications networks to provide selective call blocking or call-gapping treatment to calls to a particular telephone number or group of numbers. Situations in which such call-gapping techniques are applied include xe2x80x9ccall-insxe2x80x9d to numbers for radio or TV call-in shows that experience a concentrated volume of calling in response to contest offers or celebrity appearances.
In mobile telephone systems, events can also occur which trigger an abnormally large number of subscribers to attempt to originate mobile phone calls at the same time for processing by a common base station. This surge of system accesses in a localized area is sometimes said to result in xe2x80x9ccollisions,xe2x80x9d and the overall condition at such a base station is known as a xe2x80x9chotspot.xe2x80x9d Such hotspots can occur under extreme circumstances regardless of the number of access channels provisioned in the system. Moreover, since system accesses use the same frequency band as the traffic channels, the abnormally high system access rate increases RF interference on all carriers with access channels. This can result in the dropping of weaker calls on traffic channels using those carriers.
Merely adding access channels serving a cell (or sector, or other area), or adding more cells, may mitigate or reduce the probability of an overload condition, but will not eliminate the problem. Any location may be subject to hotspot activity in the event of a natural disaster, act of terrorism, or other traumatic unanticipated localized event. Provisioning access channels to meet such extreme overload conditions is, in any event, an uneconomic solution.
In illustrative wireless systems, a base station seeks to enhance its ability to distinguish among mobile stations simultaneously seeking to transmit messages over an access channel by using a variety of randomization techniques. One of these is known as xe2x80x9cbackoff randomizationxe2x80x9d and another is a randomization technique employing so-called xe2x80x9cpersistence testing.xe2x80x9d These randomization techniques are in addition to so-called PN randomization based, at least in part, on the identity of the calling mobile stations. Still other randomization techniques, including so-called channel randomization, are sometimes used to distribute the arrival of attempted call originations from mobile stations at a base station. While these approaches contribute significantly to the efficient use of access channels, they, too, cannot effectively deal with many hotspot conditions.
A special consideration in introducing access controls that could be imposed in emergency situations is that those mobile stations (mobiles) having an overload class programmed xe2x80x9cemergencyxe2x80x9d must continue to receive priority access to the system. In addition, any such controls should be imposed with a reasonable degree of fairness as between those non-emergency users competing for available resources.
Among the other considerations in designing access overload treatments for mobile stations is that compatibility with existing standards-based functionality must be maintained. As an example, technical industry standards that are desirably observed in introducing access controls in mobile communications system are IS-95-A+TSB Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System (March, 1995); and ANSI J-STD-008 Personal Station-Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communications Systems, (Corrected Versionxe2x80x94Aug. 29, 1995).
Limitations of the prior art are overcome and a technical advance is made in accordance with the present invention described in illustrative embodiments herein.
To maintain system integrity, and to avoid unnecessary dropping of existing calls, typical embodiments of the present invention detect access channel overload as it occurs and apply controls to maintain a sustainable rate of system accesses without affecting mobiles programmed with emergency overload class capability. More particularly, illustrative embodiments of the present invention monitor occupancy in each access channel to detect or predict an access overload condition. Such measurements advantageously use one or more service-provider-selected thresholds in determining when access channel overload control is initiated.
It proves convenient to employ access control messages from the system being accessed, e.g., a cellular or PCS wireless base station, to accessing terminals, e.g., mobile terminals, to adjust certain access parameters. In an illustrative embodiment based on the IS-95 spread-spectrum CDMA standard, it proves convenient to employ modified test parameters for known persistence tests to reflect overload conditions requiring prescribed control measures. Use of existing persistence tests typically causes mobiles of specified overload classes to incur additional random delay (including delays tantamount to blocking) before every request access probe sequence.
In accordance with present inventive teachings, illustrative overload algorithms use current occupancy readings and control settings currently in effect to determine adjustments to be made to persistence test parameters for normal mobiles (overload classes 0-9), and access channel parameters for all mobiles. Such adjustments prevent access channel usage from exceeding its allotted capacity, while avoiding unfairness as between normal mobile users.
System administration techniques advantageously provide for periodic system measurements of the number of times access channel overload control was invoked and the total duration of all overload conditions for meaningful periods (such as for each hour). Such administration techniques provide a tool for Craft to help determine what is actually happening at a site and can aid in determining the true nature of hotspot activities.